With the spread of high-definition television broadcasts and home theater systems, large-sized display devices such as PDP (Plasma Display Panel), DLP (Micromirror Projector), and the like have been put to practical use. However, the large-sized displays need the process of digitizing the input video signal into dot matrixes, and the like. Hence, a time lag (display delay time) caused until a video signal is actually displayed on a screen after the video signal is input is large, so that a delay of about 1 to 2 frame ( 1/30 to 1/15 second) occurred.
In contrast, also the audio signal needs a predetermined time in the process, e.g., the process of expanding the compressed digital signal and then converting the expanded digital signal into the analog signal, or the like, but such predetermined time is not so large as a time lag caused in displaying the video. Therefore, there existed such a problem that, when the video and the audio are reproduced as they are, the video does not synchronize with the audio because a time lag of the video is large.
Meanwhile, the technology to sense a difference in processing times in the audio/video signal processing system has been proposed (Patent Literature 1, for example). In this technology of this Patent Literature 1, a difference in processing times is measured in such a manner that the input audio/video signal in which a change in luminance and a change in sound volume occur simultaneously is processed through separate systems and then a difference between a changing timing of the sound volume of the audio signal being output after the process is ended and a changing timing of the luminance of the video signal is measured.    Patent Literature 1: JP-A-10-285483
As described above, the technology to sense a difference between a processing time of the video signal and a processing time of the audio signal in the transmission line (signal processing circuit) is disclosed in Patent Literature 1. But this Literature merely discloses the measuring technology and does not disclose the technology to measure automatically the difference and correct automatically such difference. As a result, the technology in this Literature could not be applied to the audio/video amplifier that the ordinary user employs.